The conventional, most common method to chemically synthesize .alpha.-hydroxy acids is the hydrolysis of .alpha.-hydroxy nitrile with a mineral acid such as sulfuric acid. In this case, however, a salt of the mineral acid, such as hydrogen sulfate, of more than the equivalent is produced. The mineral acid salt should be treated, resulting in a problem of the disposal of a large amount of waste.
It is also known to obtain free .alpha.-hydroxy acids by a way that a metal salt or an ammonium salt of an .alpha.-hydroxy acid, which is produced by a biological process such as fermentation of a mixture containing sugar with a microorganism or the hydrolysis of .alpha.-hydroxy nitrile with a hydrolase produced by a microorganism, is reacted with a mineral acid such as sulfuric acid or treated with an ion exchange resin. In either process, a large quantity of mineral acid salts is produced, causing the same problem mentioned above.
A method known to produce no mineral acid salts, which become waste, is the esterification of an ammonium salt of an .alpha.-hydroxy acid with alcohol (Japanese Patent Laid-Open No. Hei 7-194387), followed by the hydrolysis of the obtained ester with an acid catalyst. This method is disadvantageous in using alcohol and an acid catalyst as additional additives. These additives should be recovered. Therefore it is not an advantageous process on an industrial scale.
There is another chemical synthetic method: hydrolysis of .alpha.-hydroxy nitrile with an inorganic base such as sodium hydroxide. In this case, a mineral acid or the like should be used to neutralize to give the target .alpha.-hydroxy acid. When doing so, the equivalent of a mineral acid salt is formed, causing the same problem of waste disposal.
Furthermore, the following processes are known to prepare corresponding free acids from ammonium salts of carboxylic acids: a small amount of water is added to an ammonium salt of an unsaturated fatty acid and the mixture is heated at total reflux at 80.degree. C. or above in organic solvents in order to liberate and remove ammonia to give the unsaturated fatty acid (GB Patent Opened No. 967352); an organic solvent that forms an azeotropic mixture with water is added to a 10.about.50% aqueous solution of ammonium (meth)acrylate, and the resulting solution is heated to 60.about.100.degree. C. to distil water as an azeotropic mixture and simultaneously remove ammonia in order to obtain free (meth)acrylic acid (Japanese Patent Laid-Open No. Sho 54-115317); and a 10-80% aqueous solution of an ammonium salt of an acidic amino acid is heated while water is added to it in order to distil out ammonia and water for producing the acidic amino acid (Japanese Patent Laid-Open No. Hei 7-330696).
In these methods, ammonia is easily removed in principle if the carboxylic acid has a high dissociation constant. Their disadvantages are, however, that the degree of dissociation of ammonium ions from ammonium salts of carboxylic acids is low for strong acids with pKa below 4, such as .alpha.-hydroxy acids. Therefore it is very difficult to remove ammonia from the strong acids. To remove most ammonia, a reaction should be carried out for a long period of time or it is necessary to add a large amount of organic solvents or a great quantity of water. The inventors of this invention have applied the above three methods to prepare .alpha.-hydroxy acids, and found them inappropriate as industrial processes because 50% or more of an ammonium salt of an .alpha.-hydroxy acid remained in each of the processes.